Printed circuit boards (PCBs) are commonly formed with internal power and ground planes which are connected with surface devices such as integrated circuits mounted on the PCBs. In the operation of the PCBs, it is commonly necessary to compensate for voltage fluctuations arising between the power and ground planes in the PCBs, particularly when sensitive devices such as integrated circuits are mounted on the board and connected with the power and ground planes for operation.
Voltage fluctuations of the type referred to above are commonly caused by the integrated circuits switching on and off, the fluctuations resulting in "noise" which is undesirable and/or unacceptable in many applications.
A preferred solution to this problem has been the provision of capacitors connected directly with the integrated circuits and/or with the power and ground planes in the vicinity of selected integrated circuits. Initially, surface capacitors were formed with the surface devices or separately mounted upon the surface of the PCB and connected with the respective devices or integrated circuits, etc., either by surface traces or by through-hole connections, for example.
Surface capacitors of this type were generally effective to reduce or minimize undesirable voltage fluctuations for the devices. However, surface or bypass capacitors have not always been effective in all applications. For example, the capacitors may tend to affect "response" of the integrated circuits or other devices because the capacitors have not only a capacitive value but an inductive value as well. In this regard, it is well known that inductance arises because of currents passing through conductors such as the traces or connectors coupling the capacitors with the devices or power and ground planes.
Furthermore, the integrated circuits or other devices are a primary source of radiated energy creating noise from voltage fluctuations in the PCBs. Different characteristics are commonly observed for such devices operating at different speeds or frequencies. Accordingly, the, PCBs and device arrays as well as associated capacitors must commonly be designed to assure necessary noise suppression at both high and low speed operation.
The design of PCBs and device arrays as discussed above are well known to those skilled in the art of printed circuit board design. For purposes of the present invention, it is sufficient to realize that the use of surface mounted capacitors which are individually connected with the integrated circuits or devices substantially increase the complexity and cost of manufacture for the PCBs as well as undesirably affecting their reliability.
In order to overcome these limitations or for other reasons, a number of capacitive PCBs have been provided in the prior art. Initially, U.S. Pat. No. 4,775,573 issued Oct. 4, 1988 to Turek disclosed a multilayer printed circuit board having conductive and dielectric layers deposited on a surface of the board in order to form a bypass capacitor for devices mounted on the board.
More recently, U.S. Pat. No. 5,010,641 issued Apr. 30, 1991 to Sisler disclosed a method of making a multilayer printed circuit board with a fully cured dielectric material positioned between power and ground plane layers therein.
Still further, U.S. Pat. No. 5,079,069 issued Jan. 7, 1992 to Howard, et al. and assigned to Zycon Corporation, the assignee of the present invention, disclosed a capacitive printed circuit board including a capacitor laminate therein to provide a bypass capacitive function for devices mounted or formed on the PCB.
The printed circuit board variations disclosed above were suitable for their intended purposes. However, there has been found to remain a need for further improvements in methods for forming such PCBs and particularly for forming capacitive PCBs of the type disclosed by the Zycon patent noted above.